This invention relates generally to tracking systems, and, more particularly, to a control system capable of precision pointing of a laser beam.
Light amplification by stimulated emission of radiation (laser) has extended the range of controlled electromagnetic radiation to the infrared and visible light spectrum. A laser produces a beam of coherent electromagnetic radiation having a particular well-defined frequency in that region of the spectrum broadly described as optical. This range includes the near ultraviolet, the visible and the infrared. The coherence of the beam is particularly important because it is that property which distinguishes laser radiation from ordinary optical beams. On account of its coherence, a laser beam has remarkable properties which set it apart from ordinary light which is incoherent.
Coherence, is of two kinds: spatial and temporal. A wave is spatially coherent over a time interval if there exists a surface over which the phase of the wave is the same (or is correlated) at all points. A wave is time-coherent at an infinitesimal area on a receiving surface if there exists a periodic relationship between its amplitude an any one instant and its amplitude at later instants of time. Perfect time coherence is an ideal since it implies perfect monochromaticity, something which is forbidden by the uncertainty principle.
Laser beams have a number of remarkable properties. Because of their spatial coherence, they have an extremely small divergence and are therefore highly directional. A laser beam, because it possesses space coherence, can be focused to form a spot whose diameter is of the order of one wavelength of the laser light itself. Enormous power densities are thus attainable.
The most promising potential of lasers comes from time coherence. It is this property which permits exploitation of radio and microwaves for communications. However, laser frequencies are millions of times higher than radio frequencies, and hence are capable of carrying up to millions of times more information. In fact, one single laser beam has in principle more information carrying capicity than all the combined radio and microwave frequencies in use at the present time.
Accordingly, systems applications of lasers are useful for communication in space, on earth and underseas, as well as surveillance and weapons systems.
In many applications it is desirable to direct a laser beam at a moving object. In the prior art, moving objects were tracked by moving a telescope or other optical elements external to the laser in a manner causing the generated laser beam to follow the object. Such systems required considerable equipment in order to monitor the motion of the object and control the movable optical element accordingly. Furthermore, previous systems have used separate sampling and sensing elements; one to sample and sense the laser beam, and one to separate and sense the energy coming from the target and from the outgoing laser beam. Since samplers are generally optical elements having high loss characteristics, elimination of as many of such optical elements in an optical train without subsequent degradation of the operativeness of the system is extremely desirable.
Heretofore the elimination of essential optical elements in a tracking or pointing system required the utilization of transmissive optical elements, however, these elements could not be used with high average power lasers.
A fundamental problem with working with high average power laser beams is that all optical elements must be fitted with a heat exchanger to remove the heat generated by an absorption of the laser beam. The necessity of including a heat exchanger in the optical element forces all the optical elements which are impinged by the high average power beam to be mirrors (or more generally non-transmitters). The beam control problem, thus, is hindered by the constraint on the selection of optical elements one can choose to devise an optical system.
Previous technology includes items from two basic categories of devices; that is aperture sharing devices and beam splitters. A device which is strictly an aperture sharing device is set forth in U.S. Pat. No. 3,858,046. With such a device it is possible to transmit a high average power laser beam of one wavelength while collecting radiation in a second wavelength band. This is accomplished by using a nonuniform thickness coating to act as a lens for one wavelength and a mirror for the other wavelength. The coating is thin enough that heat can be adequately transported through the coating to a heat exchanger behind the bottom reflecting element. This method of aperture sharing does not allow beam sampling to simultaneously occur so that the target position and the beam propagation direction cannot be compared to one another.
The buried grating, another aperture sharing device, is just a high efficiency grating that is overcoated with a dielectric. The dielectric is then overcoated with a dichroic filter. The usual system implementation is to choose the dichroic filter so that the surface is reflective for the high power laser wavelength while it is transmissive to the target emission energy. Since the target emission energy is diffracted by the grating, another complimentary buried grating is normally necessary to correct the dispersion introduced by the first buried grating.
There are three beam sampling devices which have been used in the past. The most commonly used is a beam splitter. However, it cannot be utilized in high power applications because of thermal problems. The second device is a hole grating. This device is simply a mirror with fine rectangular array of circular holes. It is merely a screen and is therefore extremely crude. Even the utilization of low efficiency grating rhombs tuned to the laser wavelength have proven to be ineffective. As is therefore clearly seen, a need arises for a more reliable, efficient and economical pointing and tracking laser control system.